For data transmission systems (DTSs), a variety of performance metrics, such as symbol error rate (SER), are known to be useful. Such uses include, but are not limited to, the design, use or maintenance of DTSs.
SER can be defined as a ratio of a number of erroneously received symbols over a total number of symbols received. In the case of a binary DTS, SER can be referred to as a bit error rate (BER) that can be defined as a ratio of a number of erroneously received bits over a total number of bits received. An example level of BER, that could be regarded as acceptable for a binary DTS, is 1.0×100 errors per every 1.0×1013 bits transmitted. This BER ratio can be expressed in shorthand form as 1.0×10−13.
SER can be measured by having the transmitter of a DTS transmit a pattern of symbols known by the DTS's receiver. The receiver can compare the received symbols to the expected symbols to identify errors. The total number of symbols transmitted can be determined in two ways. First, the receiver can keep a count of the total symbols received in addition to keeping a count of erroneous symbols received. Second, a data rate between the transmitter and receiver can be known. The total number of symbols transmitted can be determined from the time period over which the transmission errors are collected (i.e., time period for collection of errors times data rate).
In addition, the ability to change, in a controlled manner, a characteristic of a DTS, such that a performance metric of the DTS (e.g., the SER of the DTS) can be measured after such change, has useful applications.
For example, a normal SER can be so low that it may not be practical to measure during normal operation of a DTS (e.g., it can take years of normal system operation to accumulate sufficient transmission errors).
An approach to estimating SER under normal operating conditions is to accelerate the errors by operating a DTS at non-normal signal levels. This non-normal SER is greater than the normal SER. The normal SER can be estimated from the following: the non-normal SER, the non-normal signal levels and the normal signal levels. Example procedures for estimating a normal SER from a non-normal SER, are disclosed in the following U.S. patent application that is herein incorporated by reference in its entirety: application Ser. No. 10/323,220, filed Dec. 18, 2002 and entitled “System and Method For characterizing the Performance of Data Communication Systems and Devices.”
Thus, there is a need for techniques of changing characteristics of DTSs, where such changes are done in a controlled way that is compatible with the measurement of performance metrics. It can be particularly difficult to achieve such controlled changes in high-speed (e.g., 6.25 Giga-bit/sec) DTSs.